Improve Spring Zeebe docs for newbies

[berndruecker]

Could be improved (raised by @ingorichtsmeier )

• Make clear that only Spring Beans can be ZeebeWorkers (@Componnent, ...)
• Improve explanation around variable handling / creating a good method signature (implement JobHandler vs. @ZeebeVariable, autoComplete, ...)
• Existing Starter and Worker examples are not sufficient / not properly designed (e.g. main method including workers) --> align with best practices e.g. shown in TwitterExample. Maybe make one project out of it.

[HanselIdes]

Recommendation: align the formatting with the downloaded format/GSG repo
Example: change zeebe.client.cloud.client-id to zeebe.client.cloud.clientId

[odranoelBR]

Hi @berndruecker .
Tks for the client, awesome!!!

I'm stuck with some points before finish a poc to present in my company, that i think this improvments will help, like :

1. How can i get current job from an instance id ? With workers is documented. Without a worker ? just an spring rest controller / service ?
2. After get this job, how can i complete it ?
3. After get this job, how can i get the form key?

[marcosdiez]

I did my part to have this issue fixed here: #449